I. Field of the Invention
The present invention relates generally to the field of biomass biorefining. More particularly, it concerns sulfite pretreatment processes to overcome recalcitrance of lignocellulose for robust conversion of biomass (SPORL) and processes to reduce energy consumption by optimizing solids-loading and size reduction.
II. Description of Related Art
Biofuel and biochemicals, such as cellulosic ethanol, have a wide use, including as an alternative liquid fuel and commodity products. If the ethanol and chemical production process only uses energy and materials from renewable sources, no net carbon dioxide is added to the atmosphere, and therefore environmental friendly. Biological conversion, such as fermentation, of biomass to ethanol is an attractive route to energy feedstocks that supplements the depleting stores of fossil fuels. Biomass is a carbon-neutral source of energy, since it comes from dead plants, which means that the combustion of ethanol produced from lignocelluloses will produce no net carbon dioxide in the earth's atmosphere. Also, biomass is readily available, and the fermentation of lignocelluloses provides an attractive way to dispose of many industrial and agricultural waste products. Finally, lignocellulosic biomass is a very renewable resource. Many of the dedicated energy crops can provide high energy biomass, which may be harvested multiple times each year.
One barrier to the production of fuel such as ethanol and chemicals from biomass is that the sugars necessary for chemical and biological conversion, such as fermentation are trapped inside the lignocellulose, a combination of lignin, hemicellulose and cellulose that strengthens biomass plant cells. Lignocellulose has evolved to resist degradation and to confer hydrolytic stability and structural robustness to the cell walls of the plants. In addition to the physical barrier of lignin, this robustness or “recalcitrance” is partially attributable to the crosslinking between the polysaccharides (cellulose and hemicellulose) and the lignin via ester and ether linkages. Ester linkages arise between oxidized sugars, the uronic acids, and the phenols and phenylpropanols functionalities of the lignin. To extract the fermentable sugars, one may need to first disconnect the celluloses from the lignin, and then acid- or enzyme-hydrolyze the newly freed celluloses to break them down into simple monosaccharides. Another challenge to biomass fermentation is the high percentage of pentoses in the hemicellulose, such as xylose, or wood sugar. Unlike hexoses, such as glucose, pentoses are difficult to ferment. The problems presented by the lignin and hemicellulose fractions are the foci of much contemporary research.
Thus, the key impediment in cellulose bioconversion is the physical and chemical barriers posed by plant cell walls that limit hydrolytic enzymes access to the biomass macrostructure (cellulose) (Eggeman and Elander, 2005; Sun and Cheng, 2002; Jeoh et al., 2007). Extensive research efforts have been devoted to various chemical pretreatments of biomass to overcome the barriers and to enhance enzyme accessibility to cellulose, by removing chemical components of biomass (lignin and/or hemicellulose). Alkaline, dilute acid, hot water, ammonia, organosolv pretreatment technologies have been developed (Mosier et al., 2005; Pan et al., 2005) with some level of success.
However, there are four major pitfalls of almost all existing chemical pretreatment processes. First, significant reduction in biomass feedstock size from chip or chop to particle of one millimeter or less (fiber or powder), is required (except for organosolv process) before chemical pretreatment can effectively remove barriers of enzyme access to cellulose to get satisfactory cellulose bioconversion efficiencies over 90%. The size reduction of biomass feedstock consumes a significant amount of electrical-mechanical or thermal energy. The size reduction is particularly critical to woody biomass because of the large native physical size and the strong integrity of wood that requires significant energy inputs to disintegrate logs into small particles. About 200-400 Watt hours (Wh) electric-mechanical energy in disk or hammer milling of one kilogram (kg) oven-dry woody biomass (Schell and Harwood, 1994; Reineke, 1961) is required, which is equivalent to 30-65% of the wood ethanol energy assuming thermal to electrical energy conversion efficiency of 30%.
The second pitfall of the existing pretreatment processes is that the enzymatic hydrolysis cellulose conversion efficiency of softwood is less than 70% (Galbe and Zacchi 2002). However, forestry is a significant source of renewable biomass feedstock for biorefining and softwood is the major woody biomass in several parts of the U.S., such as inner Pacific Northwest and Southeast, and in Canada as well as Scandinavian countries. Most forest biomass from the U.S. National Forest is mixed softwood. Efficient technologies for pretreatment of softwood for biorefining are highly desired.
The third pitfall is the limited digestibility of the substrates pretreated using the existing pretreatment processes. The slow rate of saccharification limits the productivity and yield of the bioconversion. It also requires increased enzyme loading to achieve a satisfactory cellulose-to-glucose yield. The low yield and increased enzyme dosage are two major factors that affect the economics of cellulosic ethanol and other bio-based products (Hinman et al. 1992).
The fourth is the barrier to commercialization. There is no mature equipment and technologies available for most of the existing pretreatment methods, such as reactors or extractors for steam explosion or organosolv pretreatments, and therefore capital-intensive research and development is required.
In view of these pitfalls, a practical and viable pretreatment process for conversion, such as biomass bioconversion, remains to be developed. This is especially true for woody biomass, in particular softwood, despite much research progress having been made in biomass pretreatment for bioconversion in the last several decades.